CD-type and DVD-type optical disks have been widely known as optical recording media. Especially as write-once media and rewritable media, media for which data recording can be performed at a user side, and recording apparatuses have also been spread.
For example, CD-Rs (compact discs recordable) and DVD-Rs (digiral versatile discs recordable) are typically used as write-once media. In the disks, organic pigment films are used as disk recording layers, and pits (marks) are formed by organic-pigment changes when a laser is emitted to data tracks formed as pre-grooves.
When data is recorded in such recording media, if laser power is optimized, pits are successfully formed and therefore, the quality of a reproduced signal is improved when reproduction.
To this end, when an organic-pigment-film recording medium is loaded into an optical recording apparatus, or immediately before recording is started, test writing is executed several times at a predetermined area (test-writing area) of the recording medium while laser power is slightly being changed, to determine the recording laser power which makes the quality of a reproduced signal best in the area. The asymmetry or the jitter of a reproduced RF signal is, for example, used as an evaluation function to evaluate the quality of the reproduced signal.
The most appropriate laser power is obtained before a recording operation, and so-called APC (automatic laser-power control) is applied during recording to output a laser at the most appropriate power. Therefore, a successful recording operation is allowed.
However, the obtained most appropriate laser power is just for the test-writing area.
On the disk, recording-film unevenness may occur from the center to the peripheral of the disk due to a recording-film forming process in manufacturing the recording medium.
Further, the wavelength of a laser output from a semiconductor laser device generally fluctuates according to the temperature. The wavelength of the laser emitted to the surface of a recording medium changes the optical absorption efficiency of the recording medium. In other words, even if the laser output power is constant, the energy received by the recording film of the disk is changed due to a change in laser wavelength, and therefore, the state of a pit mark generated by the energy is also changed. In summary, even if the recording laser power is output at the most appropriate value, a mark to be generated may be shifted from the most appropriate mark state.
With these points being taken into consideration, the most appropriate laser power obtained by power calibration performed at the test-writing area on the disk at a point of time is not necessarily the most appropriate recording laser power for the entire area of the disk or under every environmental condition, which includes temperature changes.
In other words, even when a laser is output by an APC operation at the most appropriate laser power obtained by power calibration, this does not necessarily mean that the most-appropriate recording operation (pit forming operation which causes a high-quality reproduction signal to be obtained during reproduction) is always implemented.
When an APC operation is performed which employs only the most appropriate recording power obtained in the test-writing area as a target value, if the quality margin of a reproduced signal is taken into account in a system, it may be effective to suppress the fluctuation of the light absorption efficiency of the recording medium as much as possible, or to employ a laser driving apparatus which has a laser in which the fluctuation of the wavelength is unlikely to occur when the temperature is changed or which has a temperature control mechanism. These measures are, however, technically complicated and disadvantageous in terms of cost.
The above problem may be avoided by proposing a format in which the deterioration of reproduced-signal quality is assumed in advance. This proposal, however, leads in an opposite direction for optical recording/reproduction systems for which higher density is strongly demanded.